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Nearby Alien Planets Not So Life-Friendly After All
By Mike Wall, Senior Writer | April 24, 2015 01:00pm ET

The new study, which was published in The Astrophysical Journal, also notes that the star Tau Ceti has much more magnesium compared to silicon than the sun does. As a result, the Tau Ceti worlds could be quite different from Earth in key ways that are not just related to size.

With such a high magnesium-to-silicon ratio, "it is possible that the mineralogical makeup of planets around Tau Ceti could be significantly different from that of Earth," said mineral physicist Sang-Heon Shim, also of ASU. "Tau Ceti's planets could very well be dominated by the mineral olivine at shallow parts of the mantle and have lower mantles dominated by ferropericlase."

Since ferropericlase is not very viscous, the rock within the mantles of Tau Ceti worlds may flow more easily than mantle rock does on Earth, researchers said. This could have potentially large impacts on volcanism and plate tectonics, the scientists added.

The current OA articles about Tau Ceti and its planets below for reference.

Tau Ceti (planetary system)

Nova Terra, Tau Ceti V
The plate tectonics are interesting on Nova Terra; currently this world has a single continent, so it is in a 'Pangaea' phase. There is no real indication of the rate of plate tectonics on this world, except for the fact that the interior is pretty much one large mountain range- a 'super-Himalayas' that covers most of the continent. That suggests to me that the rate of tectonics is fairly high.

Note, however, that the Himalayas on Earth do not include any active volcanoes- perhaps the main sites of volcanic activity on Nova Terra are in the oceans, or possibly on the plates on the fringe of the continent.
Nova Terra appears to be the planet now known as tau Ceti e, though its semi-major axis is 41.3% farther from tau Ceti and is 25.75% as massive. While Nova Terra can be explained away as a small world somehow missed by earlier surveys, this seems a bit contrived. It might be better, IMO, to retcon Nova Terra to the confirmed tau Ceti e, with the following characteristics:

Mass: 4.3 x Earth, radius: 9749 km, distance from primary: 0.552 A.U., gravity: 1.84 g, and Year: 0.464 Earth year (213.29 Nova-days), with the other entries in the data panel staying the same.

As for a high rate of plate tectonics and volcanism on Nova Terra, volcanoes are generally are located along and beside plate boundaries, whereas orogenic uplifts (folding of rock strata upwards to form mountains) typically occurs farther from those boundaries. If, as is proposed, magmatic viscosity is reduced in planets orbiting magnesium-enriched stars like tau Ceti, the main effects would be an increased rate of plate travel and a corresponding increase in the rate of emergence of "hotspot" volcanoes. Increased volcanism leads to increased emissions of sulfur oxides, carbon dioxide, and water vapor, which may, in turn, lead to increased atmospheric warming (which would be at least partly offset by the cooling effects of sulfur aerosols).

Just a few notions,

Tau Ceti e is the planet we call Elysia (the semimajor axis for this world is wrong, and may be based on early estimates). (now fixed- I think I mixed up the mass and the semimajor axis).

Nova Terra is significantly smaller than any of the planets seen so far in this system, and we currently imagine that Nova Terra has not yet been discovered. Elysia would not be a very good candidate for colonisation, at least not by nearbaseline humans. 4.3 x Earth's mass is quite a bit too massive for bipeds.
Mmm. If it's significantly less dense than Earth (more silicates and less ferrous content) you can have a planet with 4.3 Earth masses and 1.9 Earth radii. That gives you 3g at the surface. I was going to say '2g would be do-able if miserable for the first couple generations, and the grandchildren would be tweaks...' but I can't justify less than 3g with the math, and 3g would be a whole lot more no-fun than 2g. I don't think it would be wise for a 3g tweak to retain the human bodyplan. Something more like a squat centaur maybe.

Under 3g, nobody but heroically trained earthling athletes could do so much as stand up. Even if you can stand up, our lungs don't exchange oxygen fast enough to maintain a walking pace, and an uncontrolled fall would be about 20% likely to be fatal. Those are some damned horrible conditions.
(02-17-2017, 09:10 AM)Bear Wrote: [ -> ]I was going to say '2g would be do-able if miserable for the first couple generations, and the grandchildren would be tweaks...' but I can't justify less than 3g with the math, and 3g would be a whole lot more no-fun than 2g. I don't think it would be wise for a 3g tweak to retain the human bodyplan. Something more like a squat centaur maybe.

Agreed, bipedalism is hard to justify under such constraints. There's too much risk of fall damage and being upright would put a lot of stress on the vascular system. You might be able to do it walking around in a powerframe with extensive modification of the heart, lungs and probably most other organs. But if you're good enough to create aquatic humans you're good enough to make a hexapedal human that's more akin to a giant turtle than a hominid.
Mass is not as important as radius, insofar as determining habitability. As long as the radius does not exceed 1.6 Earth radii (beyond which a substantial hydrogen/helium begins to accumulate), or 10204.96 km, the planet's gravity (1.68 g for "Elysia" at that radius) is "adjustable" from a design standpoint, as none of the planets in the tau Ceti system currently has an assigned radius.

Yes, but as rocky planets get larger, they also get denser due to compaction at the core. A large, low density rocky planet is unfeasible. Even with no iron core, a rocky planet with 4.3 Earth masses would have a maximum radius of around 10,000km; this gives a surface gravity of around 1.75 gees. If the planet has any core at all it gets smaller and denser quite quickly, and the gravity increases accordingly. 1.75 is about the lowest that Elysia's gravity could reasonably be; not much more than your estimate, but still quite high (and quite unlikely).

I'm getting a radius of 9750 km for an Earth-like composition, and a surface gravity of 1.85 gees.
If Elysia is a waterworld then it could be a lot larger, and have a lower gravity; perhaps this would make it a suitable world for colonisation later in the scenario.
Note that Elysia could be much more readily colonized (especially early in the setting timeline) by one or more clades of vecs.

Here's Andrew LePage on Tau Ceti e
Quote:Tau Ceti e: MPsini=4.3 ME, Seff=1.51

The Sun-like star Tau Ceti has generated much interest over the decades among scientists looking for habitable planets. Unfortunately its relatively high level of activity has complicated efforts to find verifiable planets orbiting this star using precision radial velocity measurements. Despite the outstanding issues, one of the purported planets of Tau Ceti announced two year ago has been claimed by some to be potentially habitable. Tau Ceti e was discovered using precision radial velocity measurements but remains unconfirmed. Ignoring this issue for the moment, the analysis of the available data yields a minimum mass of 4.3 ME which appears to be near the lower end of the mass range where rocky planets transition to volatile-rich planets. Factoring in the unconstrained inclination of this planet’s orbit, there is a two in three chance that its mass exceeds the 6 ME threshold mass making it more likely to be a mini-Neptune. Its effective stellar flux also exceeds by a fair margin that for the conservative definition of the HZ. Taking all this information together, it seems that Tau Ceti e is more likely to be a hot mini-Neptune than a potentially habitable planet. These facts along with the questionable existence of this world make Tau Ceti e to be a very poor habitable planet candidate.
Even if we assume the most favourable density for this world it is still probably a bit too warm; a warm superterrestrial waterworld perhaps, or a mini-neptune.
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